Multiple control slot TDM/FDM communication system

ABSTRACT

A communication system that provides TDM and FDM communications between communication units. At least two control slots are provided in each TDM frame window to provide communication control information such as channel assignment information.

TECHNICAL FIELD

This invention relates generally to radio frequency communicationsystems.

BACKGROUND ART

Radio frequency (RF) communication systems that make use of frequencydivision multiplexing (FDM) are known. In such systems, commonly knownas trunked communication systems, communication channels are assignedfor use from time to time to subscribers as needed. To facilitate thechannel assignment mechanism, such systems generally provide either adedicated control channel that supports the channel assignmentsignalling, or the control channel function is distributed amongst thevoice channels; for example, by providing the control channelinformation subaudibly simultaneously with voice traffic.

RF systems that make use of time division multiplexing (TDM) are alsoknown. In these systems, a communication frequency (or pair offrequencies) is divided by time into frames and slots, and subscribersare assigned a slot to support their communication needs.

Combined FDM/TDM systems are also known. In such systems, a subscriberwill be assigned a particular time slot on a particular frequency (orpair of frequencies) to support its communication needs. These systemsprovide a control slot on one of the frequencies to support the channelassignment information.

The known FDM, TDM, and FDM/TDM communications systems do noteconomically support many desired features. For example, full duplex RFcapabilities must be provided in an FDM radio to support full duplexcommunications. While a properly configured TDM or FDM/TDM radio cansupport full duplex communications without full duplex RF capabilities,channel access time, channel monitoring capabilities and other featuresin such systems are often degraded.

A need exists for a communication system that will economically supportneeded and desirable features and options while simultaneously offeringhigh quality communications and operability.

SUMMARY OF THE INVENTION

These needs and others are substantially met through provision of thetime division multiplexed, multiple frequency communication systemdisclosed herein. This system supports a plurality of TDM frames on atleast some of its multiple frequencies, wherein each TDM frame includesa plurality of time slots. Some of these time slots are used tosubstantially support voice and data message communications, and two ormore of these time slots are control information slots that are used tosubstantially support control information, such as channel assignmentsignalling. Depending upon the application, the control informationslots may be on the same communication frequency, or on differentfrequencies. If on different frequencies, the control slots must stillbe in different time slots from one another to prevent contention andpreferably are not in adjacent time slots.

In one embodiment, frequency and slot assignments for voice and/or datacommunications are assigned in a way that assures that the assignmentsfor a particular communication unit do not contend with at least one ofthe control information slots, thereby assuring that all communicationunits can gain access to at least one of the control slots at all times.Further, the system can be made particularly sensitive to suchassignments, such that signalling information intended for a particularcommunication unit can be transmitted during a control slot that thecommunication unit is known to be monitoring, and not during theremaining control slots.

In another embodiment, the slot assignment process can be madedynamically responsive to loading and/or the communication needs of aparticular communication unit. In particular, the system can monitor oneor more predetermined parameters that represent loading for thecommunication system, and assign a number of time slots to a requestingcommunication unit or modify the slot duration as commensurate with themonitored parameter. For example, higher voice quality may generally beattained by assigning two time slots for communication, as versus one,thus allowing transmission of greater speech coding information. Duringtimes of light communication traffic, two time slots may be assigned toa requesting unit without degrading the grade of service offered toother subscribers. In the alternative, during times of heavy traffic, itmay be appropriate to provide a requesting unit with only one slot tosupport its voice communication traffic, thereby providing acceptableaudio quality to the requesting unit while still retaining a capabilityof servicing other units as well.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 provides a system block diagram;

FIG. 2 comprises a block diagram depiction of a repeater;

FIG. 3 comprises a block diagram depiction of a subscriber unit;

FIG. 4 comprises a timing diagram depicting a two channel/two slotsystem;

FIG. 5 comprises a timing diagram depicting outbound information in acontrol slot;

FIG. 6 comprises a timing diagram depicting inbound information on acontrol slot;

FIG. 7 comprises a timing diagram depicting a single slot simplexcommunication;

FIG. 8 comprises a timing diagram depicting a full duplex communication;

FIG. 9 comprises a timing diagram depicting an alternate full duplexcommunication; and

FIG. 10 comprises a timing diagram depicting a full duplex communicationbeing conducted simultaneously with reception of yet another signal.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring to FIG. 1, a system implementing the invention can be seen asgenerally depicted by the numeral 100. The system (100) includesgenerally a central controller (101) that controls the allocation ofcommunication channels (frequencies and time slots) to promote organizedcommunication between various communication units. The centralcontroller (101) connects via appropriate interconnections, in a knownmanner, to a plurality of repeaters (102), there being one repeater(102) for each supported frequency (or frequency pair, depending uponthe application). The central controller (101) controls the operation ofthe repeaters (102) and provides control channel information. Eachrepeater (102) may include a microprocessor and associated digitalcircuitry, in which case the controlling actions of the centralcontroller (101) may take the form of data packets transferred over aninterconnecting network (106).

The system also includes a reference timing generator (103) and one ormore auxiliary interface units (107). The timing generator (103), whichmay include a high stability reference oscillator, provides variousclock signals, including but not limited to the TDM frame clock, slotclock, and data bit clock, to the central controller (101) and therepeaters (102) to maintain time synchronization throughout the systemso that symbols, slots and frames on all frequencies coincide in time.The auxiliary interfaces (107) provide means to interconnect the system(100) with non-RF communication units, such as telephone lines anddispatch consoles.

The system also includes a plurality of TDM/FDM capable communicationunits (104). (As used herein, "communication unit" is intended to referto any of a variety of radio platforms, including but not limited tomobile radios, portable radios, and fixed location radios, includingboth one-way and two-way devices).

Referring to FIG. 2, the repeater (102) will now be described. Therepeater (102) includes a receive and a transmit section (201 and 202).The receiver section (201) includes an RF receiver (203) that receivessignals via an appropriate antenna structure (204) and that provides atits output a baseband signal. The latter signal is passed to both avoice recovery block (205) and a data recovery block (206). The voicerecovery block processes the received baseband signal to recover thevoice information signal (207). This signal might represent someprocessed version of the original audio signal. (An example of thisaudio processing can be found in a commonly owned co-pending applicationfiled April 29, 1988 and having Ser. No. 187,685, incorporated herein bythis reference.) Depending on the nature of the repeater interconnectionnetwork (106) the voice recovery block (205) may include an audioencoder/decoder to reformat the received voice signal (207). In the caseof the aforementioned audio processing, this coding function could beaccomplished with a digital signal processor, for example the MotorolaDSP56000.

The data recovery block operates in a known manner to recover any datainformation (such as, for instance, control slot information) includedin the received signal, thus providing the received data signal (208).The recovered voice signal (207) and data signal (208) are passed to arepeater network interface (209), which communicates these to thecentral controller (101) or other repeaters (102) as is appropriate viathe repeater interconnection network (106). So configured, the repeater(102) receives TDM RF signals and properly processes them to recoverboth voice and data information that may be present in thecommunication.

The transmitter section (202) also includes a network interface (210)that receives voice signals (211) and data signals (212). The voicesignals (211) comprise received voice signals (207) from the repeater'sreceiver section (201) or from the receiver sections (201) of otherrepeaters (102) in the system. The data signals (212) include controlchannel information from the central controller (101). The voice signal(211) and data signal (212) are processed by a transmitter controller(213) to provide an appropriately coded and slot and frame synchronizedsignal at its output. As in the receiver section (201), the transmittercontroller (213) may include a DSP to reformat the voice signal (211) asappropriate for various receiving communication units. The output of thetransmitter controller (213) is passed through a splatter filter (214)to an RF transmitter (215) that properly processes the signal to providea signal that may be appropriately radiated from an antenna device (216)to cause transmission of the processed signal as desired.

Referring to FIG. 3, a TDM/FDM capable communication unit (104) will bedescribed. The communication unit (104) includes an RF transceiver (301)that couples to an appropriate antenna (302) to transmit and receive RFsignals. The transceiver (301) provides a received baseband signal whichis digitized by an analog to digital converter (303). The output of theA/D converter is passed to a microprocessor (304), such as, for example,the Motorola MC68000. The baseband signal is also passed to a sync anddata recovery block (305) which processes the signal to establish frameand bit synchronization with the transmitting repeater (102). Thecommunication unit also includes a clock generator (306) which providestiming signals as appropriate to support the TDM nature of thecommunications.

The microprocessor (304) processes the received signal and passes theaudio information to a DSP coder (307), such as, for example, theMotorola DSP56000, which provides a coding/decoding function used inthis embodiment to properly encode (or decode) the audio information ina particular form. (Again, additional information regarding this form ofencoding and decoding can be found in a commonly owned co-pendingapplication filed April 29, 1988 and having Ser. No. 187,685.) In analternate embodiment, the DSP (307) could also perform the functionsprovided by the microprocessor (304) and the sync and data recoveryblock (305), resulting in a reduction of hardware complexity. Thecoder/decoder (307) couples through a filter and coder (308) to anappropriate speaker (309) and microphone (310) to allow received audioinformation to be rendered audible and to allow originating audioinformation to be processed and transmitted.

Audio information to be transmitted is passed from the DSP (307) to themicroprocessor (304) where additional information appropriate to the TDMnature of the communications is added. The resulting signal, properlycoded and slot and frame synchronized, is passed in a digitized form tothe digital to analog converter (311). The output of the D/A converter(311) is passed through a splatter filter (312) to the RF transceiver(301) which properly processes the signal to provide a signal that maybe appropriately radiated from the antenna device (302) to causetransmission of the processed signal as desired.

Referring to FIG. 4, the above described system functions in an FDM/TDMoperating environment. While certain limited applications couldeffectively use a single channel, the remaining discussion will describethe more general case wherein at least two frequencies are available forthe central (101) to assign for communication purposes, and where eachof the frequencies are subdivided into frames and slots. In thisparticular embodiment, two channels A and B are depicted, each havingframes (401) of 240 milliseconds with four slots (402) per frame (eachslot being 60 milliseconds). While in the preferred embodimentdescribed, each time slot is of uniform size, non-uniform slot sizescould be used where appropriate. Each frame window supports two controlslots (403 and 404). In one embodiment, both control slots (403 and 404)may be on the same frequency. For example, channel A may support acontrol slot in slot 1 (403) and slot 3 (404) of each of its frames(401). In another embodiment, the control slots are on differentfrequencies. For example, channel A could support a control slot in slot1 and channel B could support a control slot in slot 3. In yet anotherembodiment, the two control slots would be distributed specifically tooccur in non-adjacent slots within each TDM frame window. Thisconfiguration reduces the maximum time that a communication unit mustwait to transmit or receive system control information. For instance,for the four slot system shown in FIG. 4, the maximum time that systemcontrol information is unavailable is 60 msec, whereas if the controlslots were adjacent, this time would be 120 msec. Regardless of how thecontrol slots are apportined among the available channels, however,there will always be at least two control slots for each frame window.

A control slot supports communications control information. FIG. 5depicts the outbound signalling information (i.e., information sent bythe central (101) to the communication units (104) that may be providedin a control slot for this particular embodiment. In the 60 millisecondsprovided for the slot, 5.75 milliseconds are used to provide 69 symbolsthat represent a dotting pattern (501) (which provide atransmit-to-receive transition period) followed by 1.33 milliseconds for16 symbols that constitute a frame sync word (502) as well understood inthe art. Up to 7 outbound signalling words (OSWs) (503) can be provided,with each OSW (503) comprising 84 symbols (7 ms). Finally, 3.92milliseconds (504) are reserved at the conclusion of the slot.

The 84 symbols comprising each OSW represent 31 data bits that are errorencoded up to the 84 symbol limit. The 31 data bits themselves include16 bits for a communication unit ID, 1 bit for call type, 10 bits forfrequency assignment, and 4 bits that represent the assigned slotnumber. For example, 1 OSW could constitute a talk/request grant to aparticular communication unit by identifying that communication unit byits ID, and identifying the assigned frequency and slot.

With reference to FIG. 6, inbound signalling words (ISWs) are alsoprovided by the communication units (104 and 105) to the central (101)via the control slots. (If the channel that supports the control slotactually comprises paired frequencies (one for transmitting and one forreceiving) as often occurs in trunked communications, then ISWs and OSWscan be supported in a side-by-side manner. If the channel comprises asingle frequency only, then ISW and OSW service must be staggered toprevent contention.) For ISWs, the 60 milliseconds of the control slotare subdivided into 4 subslots (601). Each subslot (601) includes a 5.6millisecond 68 symbol dotting pattern (602) (to allow any transienteffects known to occur in the transition from receive-to-transmit tosettle) followed by 1.33 milliseconds of 16 symbol frame sync wordinformation (603). 6.5 milliseconds are then allocated to allow 78symbols of ISW information. These 78 symbols include 21 data bits thatare error encoded up to the 78 symbols. The 21 data bits include 16 bitsfor the communication unit ID and 5 bits to indicate call type request.

So configured, a communication unit (104 or 105) can request frequencyand slot assignments via the two control slots provided in each frame(401). Further, the central (101) can assign a requesting unit anappropriate frequency and slot assignment, via the two control slotsprovided in each frame (401). In an alternate embodiment, the controlslots, via particular ISWs and OSWs, could also be used to transfer datapackets between communications units (104).

Pursuant to one embodiment, when making frequency and slot assignments,the central (101) makes the assignment in a way that will assure thatthe assigned communication unit (104) will still be able to communicateon at least one of the control slots. For example, with respect to FIG.4, a requesting unit could be assigned to transmit on slot 1 of channelB and to receive on slot 2 of channel A. This would allow the assignedcommunication unit (104) to continue to monitor control information inthe control slot (404) that occupies the third time slot of channel A.In this way, important system control information, such as emergencypreemption messages, can be provided with assurance to all TDMcommunication units (104), regardless of whether such communicationunits (104) are currently engaged in communications with othercommunication units or not.

In another embodiment, the central (101) could direct controlinformation intended for the communication unit (104) previouslyassigned to slots 1 and 2 to only the control slot (404) known to bemonitored by the communication unit (104). In yet another embodiment,communication units (104) may be of differing types with differingconstraints as to how quickly a change in mode (receiving ortransmitting) or a change in frequency could be accommodated. In atypical communication system, each unit (104) can be identified by aunique ID code. The time limit described above can be associated withthe unit ID in a database maintained by the central (101). Accordingly,the central (101) would assign units so as to guarantee sufficient timeto switch from participating in the assigned voice communication tomonitoring the available control slot. In yet another embodiment,certain communication units (104), such as portable units, can benefitfrom a reduced duty cycle of receiver operation by reducing the currentdrain on the battery within the unit. To facilitate this benefit, thecentral (101) would direct control information intended for some or allof such battery operated units only to a particular system control slot,allowing these units to monitor only one control slot and thus savebattery power.

Other important benefits are available through provision of the twocontrol slots as well. For example, with reference to FIG. 7, it may benecessary or appropriate (for instance, due to heavy communicationtraffic conditions) to assign only a single time slot (701) to 2 or morecommunication units (104) to support their communication. This would, ofcourse, prevent a full duplex communication from occurring.Nevertheless, in this system, even if the assigned voice time slot (701)contends with one of the control slots (702), at least the remainingcontrol slot (703) can still be monitored by both communication units,regardless of their transmit or receive status. Therefore, if acommunication unit that is in a receive mode wishes to interrupt thetransmitting communication unit, the receive mode communication unitwill transmit an interrupt signal via an ISW on the available controlslot (703), which the central (101) then relays via an OSW in asubsequent control slot. This signal will cause the transmit modecommunication unit to cease transmitting and to begin receiving in thedesignated slot (701) regardless of its previous talk mode status. Inthis way, a receive mode communication unit can effectively interrupt areceived message and begin transmitting to the previously transmittingparty, and thereby achieve a simulated full duplex communicationcapability. While this capability is attainable with a single controlslot, the provision of two control slots significantly improves systemflexibility by allowing assignment of the communication to any slot(402) in the frame (401).

(In the above discussion, the central controller (101) is seen todetermine assignment strategies based on observed traffic conditionswithin the communications system. The traffic conditions could bemonitored in several ways, two of which will be described here forillustrative purposes. The amount of time that a communication unit(104) must wait before sufficient resources are available to assign itsrequest is related directly to traffic load. As load increases, theaverage wait time experienced by communication units increases. Thus theaverage wait time over some appropriate time interval becomes a measureof traffic load. It is also known that traffic on many communicationsystems will vary in a regular manner regularly on a daily basis.Communication systems can thus be monitored and a daily pattern oftraffic load may result. Based on this observed pattern, the trafficload can be predicted based on time of day, and assignment strategiesmodified appropriately.)

Of course, with reference to FIG. 8, if traffic conditions or otherexigencies permit, 2 time slots (801 and 802) can be made available to 2or more communication units to support their transmit and receive needs,thereby allowing full duplex communications in a half duplex RF unit(i.e., a radio that can only transmit or receive at a single moment intime). Again, as noted above, the two time slots (801 and 802) assignedto support voice activities will be assigned by the central (101) in away designed to ensure that at least one of the control slots (in thiscase, the control slot occupying slot 3 of channel A (803)) will remainuncontended by the voice channel assignment.

With reference to FIG. 9, an alternative way of supporting such a fullduplex communication would be to assign 1 time slot for voice activity(901) on channel B and a second time slot (902) to support voiceactivity on channel A. Again these assignments ensure that at least oneof the control slots (in this case, slot 3 of channel A (903)) remainsavailable to all of the communication units engaged in that particularassignment. Those skilled in the art will appreciate that multiple slotassignments can be used to facilitate other desirable features, such as,but not limited to, improved audio quality communications by permittingthe use of speech coding algorithms requiring a higher information rate.

In another embodiment, the duration of the time slots would be alteredin response to traffic conditions. For example, a standard four-slotsystem might increase slot duration by one-third to 80 millisecondsduring light traffic periods. The resulting three-slot system wouldprovide improved audio quality communications. When heavy trafficconditions occur, the slot duration could be reduced to restore fullsystem capacity.

With respect to FIG. 10, two of the time slots (1001 and 1002) could beassigned to support a full duplex voice communication as describedabove, and one or more other communication units (104) could be assignedto receive in both time slots (1001 and 1002), thereby achieving a formof conference communications where two units conduct a full duplexcommunication, while one or more other units monitor both sides of thecommunication. Further, a third slot (1003) could be assigned to one ofthe communication units to allow substantially simultaneous reception(or transmission) of a data message that may be displayed on anappropriate display mechanism in the radio. Alternatively, the thirdslot (1003) could be assigned to one of the communication units to allowsubstantially simultaneous reception of another voice message that couldbe stored in memory for future playback. Again, even while supportingthese multiple features, at least one control slot (1004) remainsavailable to all of the communications units. As a result, a singlecommunication unit can be engaged in a full duplex communication, whilesimultaneously receiving (or sending) a voice (or data) message pursuantto a different communication, and still also transmit and receive systemcontrol information as well.

We claim:
 1. A time division multiplexed communication system for allowing communications between two or more communication units, on a communication channel, wherein said system supports a plurality of TDM frames on said communication channel, and wherein each of said TDM frames includes a plurality of time slots, wherein at least some of said time slots substantially support said communications and at least two of said time slots in each TDM frame on said communication channel each substantially support independent communication control information, such that a communication unit need only receive one of the at least two of said time slots per TDM frame in order to receive communication control information relevant to that communication unit.
 2. A time division multiplexed, multiple channel communication system for allowing communications between two or more communication units, wherein said system supports a plurality of TDM frames on at least some of said multiple channels, and wherein each of said TDM frames includes a plurality of time slots, wherein at least some of said time slots substantially support said communications and at least two of said time slots each substantially support independent communication control information, such that a communication unit need only receive one of the at least two of said time slots per TDM frame in order to receive communication control information relevant to that communication unit.
 3. The communication system of claim 2 wherein said at least two of said time slots that substantially support communication control information are supported by the same communication channel.
 4. The communication system of claim 2 wherein said at least two of said time slots that substantially support communication control information are each supported by different communication channels.
 5. The communications system of claim 4 wherein said at least two of said time slots that substantially support communication control information are non-overlapping in time.
 6. The communication system of claim 2 wherein said at least two of said time slots that substantially support communication control information are non-adjacent in time.
 7. A method of communicating between two or more communication units in a time division multiplexed communication system on a communication channel that supports a plurality of TDM frames, comprising the steps of:(A) maintaining, in each of said TDM frames on said communication channel, at least three time slots, wherein at least one of said time slots is maintained to substantially support communications between said two or more communication units; and (B) maintaining at least two of said time slots in each of said TDM frames to each substantially support independent communication control information, such that a communication unit need only receive one of the at least two of said time slots per TDM frame in order to receive communication control information relevant to that communication unit.
 8. In a time division multiplexed communication system wherein at least two time slots per TDM frame are system control time slots that are used to substantially support communication control information, a method of assigning, to at least one communication unit, a plurality of time slots, comprising the steps of:(A) identifying said system control time slots; and (B) assigning said plurality of time slots to said at least one communication unit such that at least one of said system control time slots does not contend with such assignment.
 9. The method of claim 8 wherein communication control information specifically intended for at least said at least one communication unit is transmitted to said at least one communication unit during said system control time slot that does not contend with such assignment.
 10. The method of claim 9 wherein said communication control information that is specifically intended for at least said at least one communication unit is transmitted to said at least one communication unit only during said system control time slot that does not contend with such assignment.
 11. In a time division multiplexed communication system wherein a plurality of time slots are provided for each TDM frame, and wherein at least some of said time slots can be assigned to facilitate communications between communication units, a method of assigning said time slots comprising the steps of:(A) monitoring at least one predetermined parameter that is representative, at least in part, of loading for said system; (B) regulating the number of said time slots in said each TDM frame as commensurate with said monitored parameter; and (C) assigning said plurality of time slots to at least one of said communication units.
 12. The method of claim 11 wherein said predetermined parameter is the average wait time for access to a channel.
 13. The method of claim 11 wherein said predetermined parameter is the time of day.
 14. A method of substantially simultaneously transmitting and receiving voice information, transmitting or receiving data or a second voice information, and transmitting or receiving communications system control data, in a single subscriber unit, comprising the steps of:(A) in a first assigned TDM time slot, transmitting said voice information in a TDM format when there is voice information to transmit; (B) in a second assigned TDM time slot, receiving said voice information in a TDM format when there is voice information to receive; (C) in a third assigned TDM time slot, one of the following:(i) transmitting said data in a TDM format when there is data to transmit and receiving said data in a TDM format when there is data to receive; (ii) transmitting said second voice information in a TDM format when there is voice to transmit and receiving said second voice information in a TDM format when there is voice to receive; (D) and in a fourth assigned TDM time slot, transmitting communications system control data in a TDM format when there is communications system control data to transmit, and receiving communications system control data in a TDM format when there is communications system control data to receive.
 15. The method of claim 14 wherein in said fourth assigned TDM time slot, data is transmitted in a system control data format when there is data to transmit and data is received in a system control data format when there is data to receive.
 16. In a time division multiplexed communication system wherein at least one time slot per TDM frame on at least one communication channel is a system control time slot that is used to substantially support communications control information, a method of providing conference communications between at least three communication units, comprising the steps of:(A) assigning a first communication unit, via said system control time slot, a first time slot for transmitting information and a second time slot for receiving information; (B) assigning a second communication unit, via said system control time slot, said second time slot for transmitting information and said first time slot for receiving information; and (C) assigning at least another communication unit, via said system control time slot, said first and second time slots for receiving information; such that said first and second communication units can conduct a duplex communication, and said another communication unit can receive said duplex communication.
 17. In a single subscriber unit, a method of substantially simultaneously transmitting and receiving voice information, receiving another voice message, and receiving communications system control instructions, comprising the steps of:(A) in a first TDM time slot as assigned pursuant to said communications system control instructions, transmitting a first voice message; (B) in a second TDM time slot as assigned pursuant to said communications system control instructions, receiving a second voice message, which second voice message is rendered audible; (C) in a third TDM time slot, receiving said communication system control instructions; and (D) in a fourth TDM time slot as assigned pursuant to said communications system control instructions, receiving a third voice message, which third voice message is stored in memory and not rendered currently audible. 